This application claims the priority of 199 48 222.5, filed Oct. 7, 1999 in Germany, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a plate heat exchanger, comprising heat transfer plates, which exhibit a patterning stacked one above the other, and between which primary sided flow channels are formed for a first heat exchanger medium to the evaporated, and secondary sided channels are formed for a second heat exchanger heat carrier medium, wherein at least some of the primary sided and secondary sided flow channels are formed between two adjacent heat transfer plates, with patterning meshing at least partially, while maintaining a minimum spacing.
A plate evaporator for evaporating fluids with a number of stacked heat transfer plates is disclosed in the WO 91/16589. The corrugated sheet metal-like configuration of the heat transfer plates provides here between the individual plates the flow chambers for the heat exchanger mediums. To create optimal flow resistance for the fluid and the generated steam, the sweep angles of the individual flow channels along the length of the plate evaporator can be varied.
Furthermore, it is known to arrange heat transfer plates, designed in fishbone-like patterning, alternatingly and in the opposite directions, in order to produce cross channel structures. That is, in essence W-shaped fishbone-like patterning and M-shaped fishbone-like patterning are stacked one over the other. In so doing, the sweep angle is constant over the entire flow length of the plate heat exchanger in accordance with the fishbone-like patterning. Sweep angle is defined here as the angle between the main direction of flow of the heat exchanger mediums and the fishbone-like patterning of the heat transfer plates. The mediums flow in and out in the conventional manner through one borehole each, which communicates with the corresponding flow channels of the plate heat exchanger. The overall result of the alternating arrangement of w- and m-shaped, fishbone-like patterns for both heat exchanger mediums is an identical flow channel volume (identical volume on the primary and secondary side of the plate heat exchanger).
One drawback with the conventional plate heat exchangers lies in the fact that owing to the cross channel structures produced by the alternating arrangement of the fishbone-like patterning, the flow channels exhibit a relatively large volume. This leads, for example, in mediums to be evaporated to the occurrence of the Leidenfrost phenomenon, which, for example, can also be observed when a drop of water falls on a hot stove plate. Despite the thermal effect, the drop of water does not evaporate, but rather splits into a number of smaller drops.
Therefore, an object of the invention is to provide a plate heat exchanger, with which efficient evaporation can be carried out, while avoiding, in particular, the Leidenfrost phenomenon.
This problem is solved by a plate heat exchanger comprising heat transfer plates which exhibit a patterning stacked one above the other, and between which primary sided flow channels are formed for a first heat exchanger medium to be evaporated, and secondary sided channels are formed for a second heat exchanger heat carrier medium, wherein at least some of the primary sided and secondary sided flow channels are formed between two adjacent heat transfer plates, with patterning meshing at least partially, while maintaining a minimum spacing.
With the inventive plate heat exchanger it is now possible to design in particular the flow channels for a medium to be evaporated small and/or narrow so that only a small flow volume is available in particular on the primary side. This measure makes it possible to transfer the heat quite well to a medium to be evaporated, thus effectively avoiding, for example, effects like the Leidenfrost phenomenon.
Advantageous embodiments of the plate heat exchanger of the invention are described herein and in the claims.
According to an especially preferred embodiment of the inventive plate heat exchanger, the heat transfer plates are designed as plates with a fishbone-like patterning. To form the primary sided flow channels, two patternings, which run essentially in the same direction, are stacked one above the other; and to form the secondary sided flow channels, patternings, running in the opposite direction, are stacked one above the other for the purpose of producing cross channel structures. Both sides of the plates that are shaped in a fishbone-like patterning exhibit a patterning that can be used according to the invention. In stacking the essentially uniform fishbone-like patterning, two heat transfer plates can be moved very close to each other in order to form very narrow flow channels. Elevations of the one pattern mesh with the depressions of the other pattern while retaining a minimum or desired spacing. Correspondingly stacking a fishbone-like patterning, which does not run in the same direction or runs in the opposite direction, can provide a flow channel side with relatively large volume. In this case owing to the cross channel structure the result is very good heat transfer of a heat carrier medium to the heat transfer plates.
Expediently spacing elements are provided between the heat transfer plates for the purpose of adjusting the height of the flow channels. Especially in the case of heat transfer plates, whose patterning, running in the same direction, is stacked one above the other, such spacing elements can guarantee the desired and necessary minimum distance in order to provide an adequate channel diameter. With such spacing elements both the primary and the secondary sided flow channels can be optimally adapted to the concrete features. Moreover, the spacing elements have proven to be advantageous, because, as the mediums flow through the channel, they generate turbulence, thus further improving the heat exchanger properties of the plate heat exchanger.
Another preferred embodiment of the inventive plate heat exchanger provides an inlet channel, which extends through the heat transfer plates and communicates with the primary sided or secondary sided flow channels, for the purpose of introducing the heat exchanger medium into the plate heat exchanger. The embodiment also provides two outlet channels, which extend through the heat transfer plates and communicate with the primary sided or secondary sided flow channels, for the purpose of dispensing the heat exchanger medium. With these measures it is possible to achieve a very uniform flow of the heat exchanger medium inside the plate heat exchanger, thus effectively avoiding temperature gradient-induced thermal or mechanical stresses of the plate heat exchanger.
Expediently there is an inlet opening on one end of the plate heat exchanger in the region of its center axis relative to the main direction of flow. At the same time outlet boreholes on the other end of the plate heat exchanger are offset symmetrically relative to the center axis. Thus an essentially Y-shaped flow of the heat exchanger mediums can be guaranteed by the heat exchanger, a feature that results in an overall symmetrical temperature distribution. In this respect, excess thermal stress, in particular the risk of overheating, as occurs in conventional plate heat exchangers, can be effectively avoided.
According to another preferred embodiment of the inventive plate heat exchanger, a sweep angle of the patterning of the heat transfer plates is varied in the main direction of flow relative to the center axis of the plate heat exchanger. For example, decreasing the sweep angle in the flow direction of the heat carrier minimizes a pressure loss of the heat carrier. The same applies to a decreasing sweep angle in the flow direction of the medium to be evaporated.
In an advantageous improvement of the invention the primary sided and/or secondary sided flow channels exhibit a coating, with which the efficiency of the heat exchanger is improved by increasing the heat transfer area, when the coating exhibits a defined roughness.
In another design of the invention the coating of the primary sided and/or secondary sided flow channels is doped with a catalyst material, with which it is possible to generate a catalytic reaction in the heat exchanger.